A major problem confronting the use of medium energy and high energy lasers is isolation between the laser and the target. When an incident laser beam interacts with a target, a portion of the laser energy retroflects back toward the laser. This retropulse will be amplified if the amplifiers are still in the excited state. The amplified retropulse may then damage optical components near the front end of the laser chain,, notably the oscillator and pulse-switchout device. At optical and near infrared wavelengths, Faraday rotators are often used as isolators. However, at the CO.sub.2 laser wavelength, 10.6 .mu.m, Faraday rotators providing requisite extinction are not currently available.
Several devices and methods have been tried in order to isolate a CO.sub.2 laser from its target's retroreflection. Usually these involve a formation of a plasma discharge in air. The plasma reflects, refracts, scatters and absorbs laser light, to prevent transmission. One method involves the breakdown of a thin Mylar foil (Mylar is a trademark of the E. I. DuPont DeNemours and Company, a highly durable, transparent, water-repellent film of polyethylene terephthalate resin) by the laser pulse itself or by an auxiliary laser beam. Another utilizes air breakdown near a focal spot, with a reflector to concentrate the energy of the beam's fringe into the center. In this method, the beam's electric field exceeds the breakdown threshold in air. Elsewhere, air breakdowns are produced by applying an overvoltage pulse to a small region to cause a spark, the pulse being synchronized with the laser beam.